The present invention relates to a system for driving a motor and a recording apparatus provided with the motor driver.
Generally, an ink jet printer which is one of recording apparatuses is configured so that a sheet which is a record medium and which is set on a sheet feeding tray is fed by a feeder, and information is recorded on the sheet by a recording head mounted on a carriage reciprocated in a main scanning direction. The sheet is intermittently fed by a set amount in a subscanning direction by a transporter. The ink jet printer is configured so that the feeder and the transporter are driven by a sheet feeding motor, the carriage is driven by a carriage motor, and the sheet feeding motor and the carriage motor are driven by a motor driver which is disclosed in Japanese Patent Publication No. 10-323090A, for example.
In the sheet feeding motor and the carriage motor respectively arranged in the ink jet printer, the halt phase of each rotor is a so-called two-phase excitation position at which the rotor is excited by both coils of an A-phase and a B-phase, the resolution of each motor is determined depending upon the number of poles. That is, to enhance the resolution of each motor, the number of poles is required to be increased, however, there is a mechanical limit to such increase.
In the motor driver arranged in the ink jet printer, a D/A (digital to analog) converter having the resolution of 4 bits (see FIG. 15) is provided with respect to each of the A-phase and the B-phase to select one value VREF which determines current ratio of each phase to be supplied. The selection is performed such that the output torque of the motor is made constant.
Specifically, the magnitude of the output torque is represented as a synthetic vector of each current value of the A-phase and the B-phase. In a case where the motor is subjected to the W1-2phase excitation driving, each current value of the A-phase and the B-phase is varied and supplied so that a locus of the end of the synthetic vector draws a circle shown in FIG. 16.
However, as an angular interval of a vector movement on the locus at every step driving is not fixed when a current selecting circuit is selected based upon one specific value of VREF so that a circle is drawn, the precision of a halt angle is deteriorated. On the other hand, as the magnitude of the vector is not fixed when the current selecting circuit is selected so that the angular intervals of the vector movement is fixed, the precision of a halt angle is similarly deteriorated.
The motor driver arranged in the ink jet printer varies current to be supplied between, for example, 500 and 1000 mA independently with respect to the A-phase and the B-phase so that the output torque of the motor is fixed. That is, each current value of the A-phase and the B-phase is varied and supplied so that a locus of the end of the synthetic vector draws a circle shown in FIG. 16. As explained previously, the magnitude of the output torque of the motor is represented as a synthetic vector of each current value of the A-phase and the B-phase.
As magnetic attraction force for overcoming the loss of torque (load imposed on the rotor) is exercised in the two-phase excitation position, the rotor is halted at angles shown by upper points in FIG. 17. On the other hand, as the magnetic attraction force is smaller at a one-phase excitation position than that at the attraction in the two-phase excitation position, the rotor is halted at angles shown by lower points in FIG. 17. It is apparent that the angles at which the rotor is halted are dispersed as shown in FIG. 17. An angle between adjacent two-phase excitation positions is 7.5 degrees, and an angle to one-phase excitation position which is located in the middle of the two-phase excitation positions is 3.75 degrees.